Chopper circuit

ABSTRACT

A variable mark-space ratio pulse controller in which a main thyristor for controlling the supply of direct current to a load is commutated by discharge of a capacitor occuring immediately on reversal of the charge thereon so that the use of a separate thyristor which is turned on to pass discharge current from the commutating capacitor to commutate the main thyristor is avoided.

United States Patent Gurwicz [15] 3,648,151 1 Mar. 7, 1972 [54] CHOPPERCIRCUIT [72] inventor: David Gurvvicz, Gateshead, England [73] Assignee:Seveon Engineering Limited, Gateshead,

' Co. Durham, England 3,242,352 3/1966 Long ..321/4scux OTHERPUBLICATIONS Menard et al., IBM Technical Disclosure Bulletin, Vol. 6,No. 8, January 1964, pages 31, 32 a Primary Examiner-J. D. MillerAssistant Examiner-Robert J. Hickey Attorney-Browdy and Neimark 57]ABSTRACT A variable mark-space ratio pulse controller in which a mainthyristor for controlling the supply of direct current to a load iscommutated by discharge of a capacitor occuring immediately on reversalof the charge thereon so that the use of a separate thyristor which isturned on to pass discharge current from the commutating capacitor tocommutate the main thyristor is avoided.

1 CHOPPER cm'curr This invention relates to electrical control systems.More particularly, the invention is concerned with electrical controlsystems for controlling the supply of direct current to an 'electricalload.

In relation to DC electric .motors, for example, :ittis'known to controlthe mean currentsupplied in the case of aseries motor to the field andarmature windings and in the caseof a shunt motor to the fieldwindingsby means of a variablemarkspace ratio pulse controllerwhich-operates to -vary.in a given time the duration of direct-currentpulses. as the-case may be, to the field windings andarmature orto thefield windingsof the motor. Such a controller is described, for example,in our British Patent No. 950,734.

In pulse controllers of the kind referred .to it is usual practice toprovide a main thyristor for connecting the source of power to the load,the main thyristorbeing shunted by-acommutating capacitor in series witha turnoff thyristor which is itself shunted by'an inductor and aunidirectional conduction device (usually either adiode or a thyristor).The controller also incorporates oscillators for firing the thyristorsindesired timed sequence. The operation is that when the turnoff thyristoris fired, the commutating capacitor is forward charged by the directcurrent and when-the main thyristor fires it causes a power pulse toflow through the load and'it also commutates the turnoff thyristor.Moreover, during conduction of the main thyristor the charge on thecommutating capacitor is reversed by flow of charge from the capacitorvia the main thyristor, the inductor and the unidirectional'conductiondevice. The charge reversal on the commutating capacitor places it in acondition to effect commutation of the main thyristor on refiring of theturnoff thyristor.

It is an object of the present invention to avoid utilizing a turnoffthyristor in a pulse controller of the kind referred to.

The present invention consists in a variable mark-space ratio pulsecontroller which comprises a main thyristor for controlling the supplyof direct current to a load, a commutating capacitor, an inductorinseries with the capacitor, a first path in series with the capacitorand inductor and a second path disposed in parallel with the first path,wherein the first and second paths are provided by a triac device thegate of which is connected in said first path to provide uncontrolledunidirectional conduction of current through the gate and semiconductorlayers of the triac in a direction opposed to the direction of currentflow through the main thyristor, the gate of the triac devicefurtherbeing connected to a supply of triggering pulses which rendersthe triac conductive in the same direction as that of current flowthrough the main thyristor, the triac thereby functioning both as adiode in one current direction and as a controlled rectifier in theother current direction.

Suitably, the load is a DC series motor or the field windings of a DCshunt motor.

In operation prior to conduction of the main thyristor the commutatingcapacitor is charged from the source by way of the load, the inductorand the triac device in the first path.- The main thyristor then isfired into conduction so that on subsequently supplying a triggeringpulse to the triac device reversal of charge on the commutatingcapacitor takes place via the main thyristor, the second path and theinductor. When the capacitor is fully reverse charged it thenimmediately discharges viathe inductor and the triac device socommutating the main thyristor and the thyristor in the second path.

It will be appreciated that the commutation current hasto be highto'oppose the load current and since it flows through the inductor, theinductance of the inductor must below. This in turn means that thereversal time for charge on the capacitor is low and the reversalcurrent is high. Accordingly, since the commutating current is also high(as it hasto oppose the load current) the capacitor must possess arelatively high r.m.s. rating. it is only recently that the: requisitecapacitors have become commercially available and made practicable thepresent invention.

The invention will now bedescribed by way of example, withreference tothe accompanying drawing, which is a circuitdiagram of oneembodimentofthe'invention.

Referring to the drawing, the positive side of a DC power source, whichcould alternatively be a rectified AC source, is connected'via-a-maincontactor, not shown, to a series motor 5, havinganarmature 7andfieldwindings 9', which connects with .the anode of taimain thyristorllwhose cathode is conrrected to the negative side "of the-source. Themain-thyristor ll is'connected'also in parallel withaseriesrbranchcomprising acommutating-"capacitor. 13,- an inductor l5 and a triacdevice. The triac device provides first and second parallel paths ofwhich the firstpath functions as a diode permitting current flowin asense opposite to that of current flow through the main thyristorl'l,while thesecond path of the triac consists of a'thyristor which whentriggered into conduction affords aconductivepath for current whichflows in the same sense as current through the main thyristor 11. Tothis end, the triac has a gate electrode 25 which is connected via aresistor '27 to the end of the inductor 15'-remote from the comi flow ofcurrent through the thyristor 11 when the latter is in aconducting-condition. However, when a firing pulse is sup- 30 plied tothe gate 25 the 'triac is placed in a conducting state to permit currentflow in the samesense asthat through the main thyristor llduringconduction ofthe: latter. A diode 21 is providedwhich shunts-themotorand serves as a so-called free wheel" diodeto maintain'current flow inthe motor during nonconducting periods of-the main thyristor ll.

Oscillator means of known form are provided for firing th main thyristorand the triac device in known controllable manner to provide :forvariation ofth'e duration or frequency of power pulses through thecircuit of the motor 5 and main thyristorl 1.

In operation the commutating capacitor 13 charges by way of thetsource,the motor 5, the inductor l5 and the triac 23 in its uncontrolledconductive direction so that the capacitor plate connected with the mainthyristor anode is rendered positive. The main thyristor 11 is thenfired into conduction and DC power flows through the motor. A firingpulse is then supplied to the gate electrode of the triac 23 and thisenables thecharge on the capacitor to reverse by current flow'in thepath consisting'of the main thyristor 11, the triac 23 in its controlledconductive direction, and the inductor 15. As the reversal of charge iscompleted the capacitor immediately commences to discharge therebycommutating the further and main thyristors.

The period of conduction of the main thyristor is thus determined by theinterval between firingof the main thyristor 11 and the triac 23 andalso the'time taken toreverse charge the commutating capacitor 13 viathe'inductor 15. Since the commutation current has to be large (at anytime equal and opposite to the maximum load current) and since thecommutationcurrent flows through the'inductor 15, the inductance of 70prises amain thyristor forcontrolling' the supply of direct current to aload, a commutating capacitor, aninductor in series with the capacitor,afirst path in series with the capacitor and inductor, and a second pathdisposed in parallel withthe first path, wherein the first and secondpaths are provided by atriac'device, the gate of whichis connected insaid first pathto as adiode in one current direction and as a controlledrectifier in the other current direction.

2. A controller as claimed in claim 1 disposed in series with a loadcomprising one of a DC series motor and the field windings of a DC shuntmotor.

t i i t

1. A variable mark-space ratio pulse controller which comprises a mainthyristor for controlling the supply of direct current to a load, acommutating capacitor, an inductor in series with the capacitor, a firstpath in series with the capacitor and inductor, and a second pathdisposed in parallel with the first path, wherein the first and secondpaths are provided by a triac device, the gate of which is connected insaid first path to provide uncontrolled unidirectional conduction ofcurrent through the gate and semiconductor layers of the triac in adirection opposed to the direction of current flow through the mainthyristor, the gate of the triac device further being connected to asupply of triggering pulses which renders the triac conductive in thesame direction as that of current flow through the main thyristor, thetriac thereby functioning both as a diode in one current direction andas a controlled rectifier in the other current direction.
 2. Acontroller as claimed in claim 1 disposed in series with a loadcomprising one of a DC series motor and the field windings of a DC shuntmotor.